It is known that a concrete structural unit which is held in compression is capable of withstanding larger tensile and bending stresses than it can in an unstressed condition. Accordingly, in the past it has been standard practice to include within a concrete structural unit or slab elongated metal elements which are placed in tension prior to the pouring of the concrete. On setting up, the metal elements are maintained in tension due to a bond between the concrete and the elements, which, in turn, imparts a certain amount of compression to the concrete.
Since concrete structural panels are rather heavy and difficult to transport, it is desirable to fabricate the panels as close as possible to their point of final use. In the past, there have been devised various on-site techniques and apparatus for producing prestressed concrete slabs for structural units, however, all of these have been subject to one or more deficiencies such as either the need for complex and expensive apparatus, the processes were difficult to put into practice, or there was a requirement for highly skilled individuals to carry them out.